1. Field of the Invention
The present invention relates to an X-ray imaging technique.
2. Description of the Related Art
Recently, there has been developed a flat panel detector which can perform not only still image capturing but also moving image capturing (Japanese Patent Laid-Open No. 2005-253783). With recent advances in the digitalization of X-ray imaging apparatuses, there is being constructed a full digital system ranging from imaging operation to display operation. Such full digital systems sometimes use a packet communication network for the compact, versatile, and wireless configuration of transmission lines.
When using a packet communication network, the system uses a protocol designed to reliably perform retransmission such as TCP (Transmission Control Protocol) in order to reliably transmit data to a display control device. It is however difficult to increase a data rate for retransmission handling. TCP is one of several standard protocols for the Internet which are designed to perform retransmission at the time of data loss by forming a reliable connection between transmission and reception host processes. This protocol is high in reliability but low in transfer efficiency. In some cases, large packet arrival delays make it impossible to meet a request to display in real time.
For this reason, in order to reduce delays, the UDP (User Datagram Protocol) is used. However, since UDP generates no connection and performs no acknowledgment, there is a risk of packet loss, even through it is possible to implement high-speed transmission. UDP is a standard protocol for the Internet which is designed to perform connectionless communication for only transmission/reception of data between applications, and leaves reliability to the applications themselves. This protocol is low in reliability but high in transfer efficiency. As a method of solving this problem, RTP (Real-Time Transmission Protocol) in an upper UDP layer is known.
In data transmission/reception using the above communication method, packet loss sometimes occurs. As a method of solving this problem, a packet data retransmission method is available (Japanese Patent Laid-Open No. 2010-088027).
Recent X-ray imaging apparatuses have a function of holding the last image made in a previous fluoroscopy if there is an interruption or break in the fluoroscopy, and continuing to display the image on a monitor (Japanese Patent Laid-Open No. 2008-154893).
When switching from the moving image capturing mode to the still image capturing mode, the apparatus holds the last image made in the immediately preceding imaging mode. The last image at this time continues to be displayed until the execution of the next imaging operation, and hence is desired not to have much image data loss. For this reason, the apparatus prevents data loss as much as possible by taking a sufficient time for retransfer using a retransfer function.
When the user checks the state of an object in moving image capturing operation and performs still image capturing at an arbitrary timing to store an image, a retransmission time 400 for the last image is generated, as shown in FIG. 4. This may cause a delay corresponding to the retransmission time in imaging operation relative to the desired timing of still image capturing, resulting in a failure to acquire an arbitrary image.